1. Field of the Invention
This invention relates to a protective cover for photoprinting system to avoid contamination or physical damage on the surface of, e.g., photomask used in manufacturing process of LSI.
2. Description of the Prior Art
In photoprinting process for manufacturing LSI, there is used a photomask in which, for example, Cr-layer is deposited onto a glass substrate in a desired negative pattern. Photoresist coated on a silicon wafer is exposed through the photomask. Consequently, a part of the photoresist is hardend in a similar pattern to the apertures of the photomask. In such a patterning process, if there are defects such as dusts or scratches on the pattern surface of the deposited layer or the back surface of the glass substrate of the photomask or the surface of the photoresist on the silicon wafer, accurate patterning cannot be effected, resulting low yield of LSI.
In the U.S. Pat. No. 4,063,812, a photomask is sandwiched by glass plates to protect the pattern surface from dusts or scratches. By such protection, dusts or the like can but be deposited on the exterior surface of the protective glass. Thus, the images of the dusts are out of focus on the surface of the photoresist on the silicon wafer, not affecting the patterning. In this method, however, since images of defects must be made fuzzy on the desired focal plane by the locational difference between the pattern surface and the exterior surface of the protective glass, the latter has need of a considerable thickness. The required thickness of the glass is in linear proportion to the size of the defect. For example, when the latter is 10 .mu.m, it is enough that the thickness of the protective glass is about 1 mm. But the defect of 50 .mu.m requires the protective glass of about 5 mm thickness. If the thickness of the glass increases, as a matter of course, the absorption of light therein also increases. This results long exposure time for the patterning or large luminance brightness of the light source. The increase of the luminance brightness of the light source, however, causes, in addition to technical problems and enormousness of the system, considerable heat which results shortening of the machine life. The elevation of the temperature also results the displacement of the pattern or physical damage thereto due to the thermal expansion of the photomask. This problem will be avoided if a glass composition with low absorption of light is used. In that case, however, because such a composition is peculiar and so the glass having that composition is not mass-produced industrially, the latter becomes expensive. Besides, in such glasses, there is fluctuation of quality such as accuracy of dimension and homogeneity. If the thickness of the protective glass increases, another problem arises in addition to the above-described absorption problem. That is, because the difference of the refractive index due to the inhomogeneity in the glass is accumulated, distortion of the pattern image on the photoresist becomes considerable.
Recently, as integration of LSI is advanced, it has been required that the width of pattern line on silicon wafer is more decreased. For this purpose, the wavelength of light for exposure must be shortened and so light having the wavelength in ultraviolet range is used. In that case, however, absorption of light in glass is remarkably increased. Accordingly, it has been desired that the absorption of light in the protective glass is decreased even if special glass such as ultraviolet ray transmitting glass is used.
Pattern image projected on the photoresist contains an image with rays which passed through the apertures of the photomask without reflection and another image with rays which passed out of the apertures after being reflected by the boundaries between the interior and exterior of the aperture. After being introduced in the aperture, the latter rays are reflected first by the near side boundary to the photoresist and then by the remote side boundary and subsequently passed out of the aperture. On the optical axis, the above both images perfectly coincide with each other, resulting no problem. Out of the optical axis, however, there appears locational difference between them on the photoresist surface so the pattern image is confused. If the thickness of the aperture, that is, thickness of the photomask is in order of .mu.m, that confusion is tolerable because the locational differnce between the both images is little. On the other hand, if the photomask is considerably thick, e.g., more than 5 mm, the image with the reflected rays is out of focus on the photoresist surface because of optical-path difference, resulting no problem. If the thickness of the photomask is within the range between scores .mu.m and several mm, however, the image with the reflected rays is made in the vicinity of the photoresist surface so the confusion of the pattern image becomes remarkable.
In the U.S. Pat. No. 4,131,363, there is described a pellicle cover for projection system, in which a pellicle made of organic resin and having the thickness of several .mu.m is stretched over a ring-like spacer. Because the pellicle is very thin, absorption of light terein is little and confusion of the pattern image due to the image with the reflected rays is ignorable. Such pellicle, however, is very delicate and breakable because it is made of thin organic material. Moreover, static electricity tends to generate on the pellicle so dusts or other contamination particles are apt to adhere thereto. Although the pellicle cover is to make the location of adherence of the dusts differ from that of the pattern surface, a large number of dusts adhering to the pellicle cover affect the pattern image as a matter of course. Accordingly, cleaning of the cover is necessary and so the pellicle may be scratched or broken in the cleaning process. In the case that ultraviolet rays are used for exposure, the material of the pellicle is deteriorated by irradiation with the light of the short wavelength so that the transparency of the cover becomes bad and the pellicle becomes brittle to shorten the life thereof. Besides, the pellicle expands considerably because of the large expansion coefficient, which causes looseness of the pellicle or rumples thereon.